I have this bag and it's the bomb:
Deuter Traveller 55+10
There are two different varieties of backpacks - camping/hiking bags and "travel" style bags. Most camping/hiking bags have a single opening in the top (usually drawstring with a flip top) and this style cannot usually be locked. They are also a bit limiting as you have to often pull everything out to get at the items in the bottom. However, on the flip side, they often have more sophisticated suspensions systems so can be more comfortable to wear for long distances, etc.
I prefer a "travel" style bag like the one I bought - it has a zippable cover for the straps and can be padlocked for checking on an airplane, the bag opens entirely on the side so you can get at things more easily, etc. I highly recommend it!
Cheers
Mark